Most conventional sports rackets have strings which are effectively anchored as they pass across the head or face opening to the next string hole. String elasticity, static string tension, frame stiffness, strung area geometry, and balance are all factors known to affect power, control, comfort and equipment performance. Many shapes, sizes, material, string tension, weights, even grips have been tried to improve the various games.
For example, U.S. Pat. No. 3,999,756 to Head suggests enlargement of the strung area for tennis rackets to enhance physical principles such as location of the center of percussion and magnitude of the polar moment of inertia. The Head patent does not suggest change of the intrinsic physics of the ball/string/frame collision. The racket head is shown with each string chord being a different length than its neighbor. Each string is effectively anchored at opposite ends as it bends over a resilient pad on the frame and extends to the next adjacent hole. The anchored string relationship will result in high dynamic tension along the individual string cords during impact with resulting problems of string and racket breakage, sensitivity to weather, and wear. For a given racket size and shape, the player is left with a choice of how much tension to use on which brand of string, depending upon the player's style.
Placement of elastomers or springs between the strings and the frame have the same basic affect as a reduced static tension applied to the strings. The essential nature of the dynamic response is not changed.
U.S. Pat. No. 1,559,986 to Quick, U.S. Pat. No. 4,203,597 to Reedhead et al, and U.S. Pat. No. 4,462,592 to Legger et al, all exemplify spring mounting devices for tennis racket strings. They make use of various pulley or guide arrangements mounting the string cords to various forms of springs to which tension along the strings is transmitted. The springs function as reaction members with responses varying with the spring resistance to compression or extension. These patents are also illustrative of slides or sheave arrangements guiding the string courses across the racket face. While sheaves have some effect on transmission of tension from one string chord to the next, the effect is diminished significantly by the spring action.
U.S. Pat. No. 4,441,712 to Horst Guthke illustrates a tennis racket string mount. Strings are connected to the racket frame through a number of linking elements that permit displacement of the string connection points relative to the racket head frame. A complex linkage and pin arrangement is used to eliminate asymmetrical deflection of the strings in the area adjacent the head frame. The linkages are used to essentially change the anchor point of the various strings in relation to the racket head frame. The result claimed is establishment of the advantages of a large racket head in a conventional size head frame. However, the string connections extend into the string opening of the racket or otherwise effectively shorten the string dimensions extending across the racket face. Reduced string length has an effect on useful or "dynamic" string tension. Furthermore, the various pivot arrangements lose efficiency through friction at the several pivot points. Pulleys or sheaves within the racket face interconnect a single strand of string arranged through the various pulleys to form the strung racket face. The pulleys extend into the string opening and therefore effectively shorten the overall string length. They therefore decrease the opening size. Furthermore, the pulley and link arrangements add significantly to the head weight of the racket.
While the above have attained limited success, there remains a need for optimizing power and control features of a strung racket while minimizing the complexity of mechanisms for achieving such results. It is also desirable to incorporate simple, yet effective, features in a strung racket that will produce a higher coefficient of restitution (increased power for the same exertion), lower stress in the racket frame during the hit (lower magnitude vibrations), lower peak tension in the strings during the hit, balance restoring forces by providing even dynamic string tension seen by the hit object (approaching that of a uniform membrane), reducing edge effects on "off-center" hits, and producing a longer dwell time for better control and larger string deflection for additional power.